CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
medications and other concurrent conditions that may cross-react with the procedure
and give a falsely positive result.
Predictive value is not the only criterion of laboratory test usefulness and
may at times be misleading if used too rigidly. For example, a test may have
excellent characteristics as a screening procedure in terms of sensitivity, low cost,
and ease of technical performance but may also have a low positive predictive
value. Whether the test is useful would depend on other factors such as the
type and cost of follow-up tests necessary in case of an abnormal result and the
implications of missing a certain number of persons with the disease if some less
sensitive test were used.
Sometimes, reasons for circumstances where predictive value are misleading
or difficult to establish. If the medical laboratory worker is calculating the
predictive value of a test, he or she must first know the sensitivity and specificity
of that test. This information requires that some accurate reference method
for diagnosis must be available other than the test being evaluated. A standard
against which the test in question can be compared is sometimes called a “gold
standard.” An example of a gold standard is the use of a complex procedure
called an electrophoresis for total protein. This test would not be used for normal
testing procedures but could be used to evaluate a simple and quick method
such as dye-binding. In some cases, this is not possible. There may not be a more
sensitive or specific test or test combination available or the test being evaluated
may itself be the major criterion by which the diagnosis is made. In other words,
if it is not possible to detect all or nearly all patients with a certain disease, it will
not be possible to provide a truly accurate calculation of sensitivity, specificity, or
predictive value for tests used in the diagnosis of that disease. The best one can
obtain are estimates, and these vary in their reliability.
Calculation of Predictability Values
This is most often achieved by computer analysis, either by automated entry of
data or by manual entry of data by the technician or technologist. The student
should become familiar with the computer terms commonly used in the process.
Almost all laboratory equipment now has computer processes for operating the
mechanical aspects of the procedure, as well as interpreting the results from raw
data. Patient values can then be read as meaningful figures before transmitting
them to the appropriate clinical areas for use in diagnosing and treating the
patient. QC records typically are recorded for long-term management in the
Laboratory Information System (LIS) for reference when errors such as shifts
and trends occur. Repeat occurrences of the same or similar problem might herald
a pending problem of a certain type.
What Is Meant by Reproducibility and Accuracy?
Reliability of laboratory tests may be greatly affected by the level of technical performance
within the laboratory. The effect of these technical factors is reflected
in test reproducibility and accuracy. Reproducibility (precision, or inherent or
built-in error) is a measure of how closely the laboratory can produce the same
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